Unless otherwise indicated herein, the materials described in this section are not prior art to the claims and are not admitted to be prior art by inclusion in this section.
In a wireless communication system, a base station may transmit downlink data to one or more wireless communication devices (WCDs) that are operating in a wireless coverage area provided by the base station. The wireless coverage area could be, for example, a cell or a sector. The base station may also receive uplink data from one or more WCDs operating in the wireless coverage area.
Some of the downlink data transmitted by the base station may be transmitted in a downlink channel that can be shared among multiple WCDs. For example, the Long Term Evolution (LTE) of the Universal Mobile Telecommunications System (UMTS) defines a Physical Downlink Shared Channel (PDSCH) as the primary downlink channel for transmitting user data to WCDs. In addition, LTE defines downlink control channels that carry various types of control signaling. The downlink control channels include a Physical Control Format Indicator Channel (PCFICH), a Physical Hybrid ARQ Indicator Channel (PHICH), and a Physical Downlink Control Channel (PDCCH).
In the LTE approach, downlink resources are mapped in the time domain and in the frequency domain. In the time domain, LTE defines 10 millisecond (ms) frames, 1 ms sub-frames and 0.5 ms slots. Thus, each frame has 10 sub-frames, and each sub-frame has 2 slots. In the frequency domain, resources are divided into groups of 12 sub-carriers. Each sub-carrier is 15 kHz wide, so each group of 12 sub-carriers occupies a 180 kHz bandwidth. The 12 sub-carriers in a group are modulated together, using orthogonal frequency division multiplexing (OFDM), to form one OFDM symbol.
LTE further defines a particular grouping of time-domain and frequency-domain resources as a downlink resource block. In the time domain, each downlink resource block has a duration corresponding to one sub-frame (1 ms). In the frequency domain, each downlink resource block consists of a group of 12 sub-carriers that are used together to form OFDM symbols. Typically, the 1 ms duration of a downlink resource block accommodates 14 OFDM symbols, each spanning 66.7 microseconds, with a 4.69 microsecond guard band (cyclic prefix) added to help avoid inter-symbol interference. Depending on the bandwidth of the system, multiple downlink resource blocks can be transmitted in each 1 ms sub-frame. For example, a system with a 5 MHz bandwidth may be able to transmit 25 downlink resource blocks in each 1 ms sub-frame. Whereas, a system with a 15 MHz bandwidth may be able to transmit 75 downlink resource blocks in each 1 ms sub-frame.
The smallest unit of downlink resources is the resource element. Each resource element corresponds to one sub-carrier and one OFDM symbol. Thus, a resource block that consists of 12 sub-carriers and 14 OFDM symbols has 168 resource elements. Further, each OFDM symbol and thus each resource element can represent a number of bits, with the number of bits depending on how the data is modulated. For instance, with Quadrature Phase Shift Keying (QPSK) modulation, each modulation symbol may represent 2 bits; with 16 Quadrature Amplitude Modulation (16QAM), each modulation symbol may represent 4 bits; and with 64QAM, each modulation symbol may represent 6 bits.
Within a resource block, different resource elements can have different functions. In particular, a certain number of the resource elements (e.g., 8 resource elements) may be reserved for reference signals used for channel estimation. In addition, a certain number of the resource elements (e.g., resource elements in the first one, two, or three OFDM symbols) may be reserved for control signals in the PCFICH, PHICH, and PDCCH channels. In the frequency domain, the resource elements that define these control channels in each sub-frame cooperatively span all of the resource blocks across the bandwidth, leaving most of the remaining resource elements in the resource blocks available for use to carry user data in the PDSCH channel.
In practice, the PCFICH indicates how many OFDM symbols (how many symbol time segments) in each sub-frame are reserved to define the control channels. Further, of the resource elements in the indicated OFDM symbols, a particular set of the resource elements may be set aside to define the PCFICH and PHICH, leaving the remaining resource elements for use to define the PDCCH. For example, 16 resource elements in the first OFDM symbol may define the PCFICH, and a certain number of resource elements in the indicated OFDM symbols may define the PHICH. Consequently, the size of the PDCCH may depend largely on how many OFDM symbols are reserved to define the control channels.
One of the main functions of the PDCCH is to carry downlink control information (DCI) messages to served WCDs. A DCI message spans a particular set of resource elements on the PDCCH (e.g., one, two, three, or four control channel elements (CCEs), each including 36 resource elements) and provides a served WCD with information indicating how the WCD should receive data on the PDSCH in the same sub-frame, or how the WCD should transmit data on an uplink shared channel in an upcoming sub-frame. For instance, a DCI message may specify particular resource blocks that carry data to the WCD on the PDSCH, what modulation scheme is used for the transmission, and so forth. Further, each DCI message may include a cyclic redundancy check (CRC) that is scrambled with an identifier (e.g., cell radio network temporary identifier (C-RNTI)) assigned to the WCD, so that the WCD can identify and read the DCI message.